1. Field of the Invention
The present invention relates generally to a color temperature control circuit for a color television receiver, and is directed more particularly to a color temperature control circuit for a color television receiver in which the blue primary color voltage is made larger than the red primary color voltage when the ratio between an output by an input is nearer the white level more than a certain level.
2. Description of the Prior Art
In order to reproduce an image on a color television receiver with the complete color fidelity and also beautiful white color at the white peak, it is necessary that the reference white of the television receiver (cathode ray tube) is made different between the time when the luminance level of a video signal is low and the time when the luminance level is high. For example, it is desired that at a level lower than the flesh color level, a color temperature is held at 9300.degree. K.+8 MPCD (minimum perceptible color difference) while made at 14000.degree. K.+8MPCD at the white peak level.
In the art there has been proposed an apparatus to control the color temperature of a cathode ray tube as shown in FIG. 1. In this prior art control apparatus, red, green and blue primary color signal voltages R, G and B derived from a color decoder 10 are respectively supplied through a video output circuit 20 and through red, green and blue cathode resistors R.sub.R, R.sub.G and R.sub.B to control electrodes or cathodes K.sub.R, K.sub.G and K.sub.B of a tri-color cathode ray tube 30. In this case, a series connection of a zener diode Z.sub.G and a resistor r.sub.G is connected in parallel to the green cathode resistor R.sub.G and a series connection of a zener diode Z.sub.B and a resistor r.sub.B is also connected in parallel to the blue cathode resistor R.sub.B. Accordingly, when the green and blue cathode currents increase, the zener diodes Z.sub.G and Z.sub.B become each conducted, and hence the green and blue cathode feedback resistances are reduced from R.sub.G and R.sub.B to R.sub.G //r.sub.G and R.sub.B //r.sub.B respectively. Thus, the green and blue feedback amounts decrease, and the green and blue drive currents increase whereby the color temperature becomes high.
In this case, however, the feedback resistance is the sum of the output impedance of the video output circuit 20 and the above resistance. Therefore, even if the resistances are varied from R.sub.G and R.sub.B to R.sub.G //r.sub.G and R.sub.B //r.sub.B, the variation of the feedback resistance is less. For this reason, the color temperature is less in variation and hence it is rather difficult that a desired color temperature is presented at a white peak level as indicated by a solid line 1 in the graph of FIG. 2, and the color temperature is insufficient as indicated by a dotted-line 2 in the graph of FIG. 2. If the feedback resistance is varied at a lower luminance level to thereby vary the color temperature, the desired color temperature can be obtained at the white peak level as indicated by a dotted-line 3 in the graph of FIG. 2. However, in order to repreduce the color with high fidelity, it is necessary that the color temperature is kept at a low temperature until the flesh color level. Therefore, the above measure is not so desired.
Another meansure is also proposed, by which the series connections of the zener diodes Z.sub.G, Z.sub.B and resistors r.sub.G, r.sub.B shown in FIG. 1 are not used but the resistance values of the cathode resistors R.sub.R, R.sub.G and R.sub.B themselves are made different as R.sub.R &gt;R.sub.G &gt;R.sub.B. This measure proposes the defect similar to the former measure. In addition, it is not desired to select the resistance values of the resistors R.sub.G, R.sub.B low in view of protecting the video output circuit 20 from the discharge in the cathode ray tube 30. Further, when the differences among the resistance values of the resistors R.sub.R, R.sub.G, R.sub.B are large, the frequency characteristics among the red, green and blue primary colors do not coincide, the pulse characteristic is deteriorated and a color smear is generated.